Powder coating compositions and process for the manufacture thereof

ABSTRACT

Compositions suitable for application in powder coating processes are produced by melt atomisation of thermosettable mixtures. The product is characterised by improved particle size distribution and by a generally rounded particle shape.

This invention relates to powder coating compositions and to theirpreparation.

BACKGROUND OF THE INVENTION

Powder coatings form a rapidly growing sector of the coatings market.Powder coatings are solid compositions which are generally applied by anelectrostatic spray process in which the powder coating particles areelectrostatically charged by the spray gun and the substrate is earthedor oppositely charged. The composition is then heated to melt and fusethe particles and to cure the coating. The powder coating particleswhich do not adhere to the substrate can be recovered for re-use so thatpowder coatings are economical in use of ingredients. Also, powdercoating compositions are generally free of added solvents and, inparticular, do not use organic solvents and are accordinglynonpolluting.

Powder coating compositions generally comprise a solid film-formingresin, usually with one or more colouring agents such as pigments. Theyare usually thermosetting, incorporating, for example, a film-formingpolymer and a corresponding curing agent (which may itself be anotherfilm-forming polymer). Powder coating compositions are generallyprepared by intimately mixing the ingredients, for example in anextruder, at a temperature above the softening point of the film-formingpolymer(s) but below a temperature at which significant pre-reactionwould occur. The extrudate is usually rolled into a flat sheet andcomminuted, for example by grinding, to the desired particle size. Theparticle size distribution required for most commercial electrostaticspray apparatus is between 10 and 120 microns, with a mean particle sizewithin the range of 15 to 75 microns, preferably 25-50 microns.

Whilst existing processes for the manufacture of powder coatingcompositions are satisfactory in some respects, there is neverthelessroom for improvement and it is a general objective of the presentinvention to simplify the production of such compositions and also tomake such production more economic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic cross-section of an apparatus for performing aprocess according to the invention;

FIG. 2 is a cross-section of one form of rotary cup atomizer suitablefor use in the apparatus of FIG. 1;

FIG. 3 is a cross-section of an alternative form of rotary cup atomizersuitable for use in the apparatus of FIG. 1;

FIG. 4 is a cross-section, partly in a diagrammatic form, of anapparatus including a modified form of atomizer provided with anadditional hot air inlet;

FIG. 5 is an electron micrograph of a sample of the product of theinvention; and

FIG. 6 is an electron micrograph of a sample of the product obtained bya conventional comminution process.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for the manufacture of athermosetting powder coating composition, which comprises forming amolten mixture of a film-forming polymer containing reactive functionalgroups, a curing agent for the said polymer and optionally one or morecolouring agents, atomising the resulting melt into droplet form, andcausing or allowing the atomised droplets to cool to form solid powderparticles, the conditions being controlled to avoid significantthermosetting of the composition during the process, whereby the productcomposition is suitable for application as a powder coating.

It is a specific objective of the process of the invention to produce,without comminution, a composition which is suitable for application asa powder coating, and it will be appreciated that the attainment of thatobjective depends on the prevention of any significant thermosetting ofthe composition during the process. On the basis of the present state ofknowledge, it could not have been expected that the necessaryperformance criteria for a powder coating composition could be achievedby any process involving atomisation of a molten thermosetting materialand no such process would have come into consideration on the basis ofprevious knowledge or proposals. Thus, it would on the contrary havebeen expected that the temperature conditions required in order to bringthe composition to a form in which it could be atomised would inevitablyresult in significant thermosetting of the composition; that is to say,it would have been expected that thermosetting would proceed to a degreewhich would produce an unacceptably detrimental effect on the product.

By avoiding the need for a comminution step, the process of the presentinvention achieves its general objective of simplifying the manufactureof powder coating compositions. The process of the invention at the sametime avoids certain disadvantages in terms of the size distribution andshape of particles produced by previously proposed comminutionprocesses. In terms of particle size, the product of the presentinvention may be controlled to have a significantly narrower variationin size as compared with comminution products, and the invention mayalso be used to reduce the proportion of undesired undersize particles.

In terms of shape, the process of the invention tends to produceparticles which are rounded and approximate more closely to sphericalshape, as distinct from the much more angular (for example, acicular)particles produced by conventional comminution techniques. The fact thatthe product particles are more rounded in shape is believed to haveadvantages in terms of retention of electrostatic charge in powdercoating application processes. Thus, it would be expected that therewould be significantly more charge leakage from an acicular or otherangular particle.

The greater regularity of shape, in combination with narrower particlesize distribution, obtainable by the process of the invention isconsidered to result in a product which is more readily fluidizable,especially at low particle sizes, than products obtained by comminution.

In terms of aspect ratio it should be noted that the aspect ratio of theproduct particles will in general approximate more closely to sphericalform than those produced by comminution processes, and generally atleast 70% of the particles will have an aspect ratio (length to smallestdiameter) of less than 2:1, as determined by optical microscopy.

The process of the invention has the further advantage that the powdercoating particles produced generally have a surface which consistssubstantially wholly of film-forming polymer. Colouring agents and otheradditives are encapsulated within the particle. This allows more rapidand even fusion when the powder coating is cured on the substrate.

It has also been observed that, at least in some instances, a proportionof the powder coating particles produced by the process of the inventionmay have a hollow core. The use of such hollow particles offers thepossibility of producing thinner coating films than those produced bysolid particles of similar dimensions when applied under similarconditions.

The film-forming polymer used in the manufacture of a thermosettingpowder coating composition according to the invention is advantageouslyother than a phenolformaldehyde resin and may be one or more selectedfrom carboxy-functional polyester resins, hydroxy-functional polyesterresins, epoxy resins, and functional acrylic resins.

As already noted, powder coating compositions do not use organicsolvents, and it is an important feature of the process of the inventionthat substantially no solvent is to be added to the composition at anystage, and moreover that the content of residual solvent derived fromthe manufacture of any of the ingredients should be as low as possible.Thus, the residual solvent content of the composition prior toatomisation should advantageously be no more than 1.0% by weight,preferably no more than 0.5% by weight, and more especially no more than0.3% by weight.

The powder coating composition can, for example, be based on a solidpolymeric binder system comprising a carboxy-functional polyesterfilm-forming resin used with a with a polyepoxide curing agent. Suchcarboxy-functional polyester systems are currently the most widely usedpowder coatings materials. The polyester generally has an acid value inthe range 10-100, a number average molecular weight Mn of 1,500 to10,000 and a glass transition temperature Tg of from 30° C. to 85° C.,preferably at least 40° C. The poly- epoxide can, for example, be a lowmolecular weight trifunctional epoxy compound such as triglycidylisocyanurate (TGIC) or an epoxy resin such as a condensed glycidyl etherof bisphenol A. Such a carboxyfunctional polyester film-forming resincan alternatively be used with a bis(beta-hydroxyalkylamide) curingagent such as tetrakis(2-hydroxyethyl) adipamide.

Alternatively, a hydroxy-functional polyester can be used with a blockedisocyanate-functional curing agent or an amine-formaldehyde condensatesuch as, for example, a melamine resin, a urea-formaldehyde resin, or aglycol ural formaldehyde resin, for example, the material "Powderlink1174" supplied by the Cyanamid Company, or hexahydroxymethyl melamine. Ablocked isocyanate curing agent for a hydroxy-functional polyester may,for example, be internally blocked, such as the uret dione type, or maybe of the caprolactam-blocked type, for example, isopheronediisocyanate.

As a further possibility, an epoxy resin can be used with anamine-functional curing agent such as, for example, dicyandiamide.Instead of an amine-functional curing agent for an epoxy resin, aphenolic material may be used, preferably a material formed by reactionof epichlorohydrin with an excess of hisphenol A (that is to say, a polyphenol made by adducting bisphenol A and an epoxy resin). A functionalacrylic resin, for example a carboxy-, nydroxy- or epoxyfunctional resincan be used with an appropriate curing agent. Mixtures of binders can beused, for example a carboxy-functional polyester can be used with acarboxy functional acrylic resin and a curing agent such as abis(beta-hydroxyalkylamide) which serves to cure both polymers. Asfurther possibilities, for mixed binder systems, a carboxy-, hydroxy- orepoxy-functional acrylic resin may be used with an epoxy resin or apolyester resin (carboxy- or hydroxy-functional). Such resincombinations may be selected so as to be co-curing, for example, acarboxy-functional acrylic resin co-cured with an epoxy resin, or acarboxy-functional polyester co-cured with a glycidyl-functional acrylicresin. More usually, however, such mixed binder systems are formulatedso as to be cured with a single curing agent (for example, use of ablocked isocyanate to cure a hydroxy-functional acrylic resin and ahydroxy-functional polyester). Another preferred formulation involvesthe use of a different curing agent for each binder of a mixture of twopolymeric binders (for example, an amine-cured epoxy resin used inconjunction with a blocked isocyanate-cured hydroxy functional acrylicresin).

A powder coating composition of the invention may be free from addedcolouring agents, but usually contains one or more such agents (pigmentsor dyes) and can contain additives such as a flow-promoting agent, aplasticiser, a stabiliser, for example a stabiliser against UVdegradation, an anti-gassing agent, such as benzoin, a filler, or two ormore such additives may be present in the coating composition. Examplesof pigments which can be used are inorganic pigments such as titaniumdioxide, red and yellow iron oxides, chrome pigments and carbon blackand organic pigments such as, for example, phthalocyanine, azo,anthraquinone, thioindigo, isodibenzanthrone, triphendioxane andquinacridone pigments, vat dye pigments and lakes of acid, basic andmordant dyestuffs. Dyes can be used instead of or as well as pigments.

A pigment content of <40% by weight of the total composition may beused. Usually a pigment content of 25-30% is used, although opacity canbe obtained with dark colours with <10% by weight of pigment. Whereappropriate, a filler may be used to assist opacity, whilst minimisingcosts.

In many cases, it will be preferable for the powder coating compositionto include a small quantity of a catalyst for the curing reaction.Preferably, however, to prevent or at least reduce undesirablepre-reaction, the catalyst is not incorporated in the composition beforeor during the melt mixing step of the process. Instead, the catalyst isadvantageously introduced, preferably in liquid form, immediately beforethe flowing melt reaches the atomiser or is injected directly into theatomiser itself, so as to be mixed with the molten film-forming polymerbefore the onset of droplet formation. Some of the materials suitable ascatalysts are liquid at room temperature; others will require heating torender them liquid. Examples of suitable catalysts include stannousoctoate, dibutyltin laurate, triphenylphosphine, imidazoles such as thatsold under the trade name "Curezol", phenylimidazoline, tertiary aminessuch as, for example, benzyldimethyl amine, tetrabutylammonium bromide,and amines blocked with boron trichloride (for example, the materialknown as DY 9577 available from Ciba-Geigy AG). Mention should also bemade of phosphonium salts, more especially phosphonium halides, forexample, benzyltriethyl phosphonium chloride or triphenylethylphosphonium bromide. It will be appreciated that some of the foregoingmaterials listed as catalysts may also serve as curing agents in thecase of certain polymers, especially oxirane-containing polymers.

In certain cases, it may be possible and advantageous to incorporate thecatalyst in the form of a masterbatch comprising polymer and catalyst.Thus, for example, the catalyst may be dispersed in a polymer (forexample, a polyester) with which it does not react, and the resultingmasterbatch may be heated to form a melt before being incorporated withthe remainder of the thermosetting composition immediately beforeatomisation.

In a typical process according to the invention, the curing agent forthe film-forming polymer will be incorporated in a melt-mixing operationbefore the melt reaches the atomiser. In principle, however, as in thecase of the catalyst referred to above, at least a proportion of thecuring agent may be introduced immediately before the flowing meltreaches the atomiser or may be injected directly into the atomiseritself, so as to be mixed with the molten film-forming polymer beforethe onset of droplet formation.

Formation of the molten composition mixture may be carried out in anysuitable melt-mixing apparatus, for example, a triple-roll mill, aBanbury mixer or a Z-plate mixer. Preferably, however, the mixing iscarried out in an extruder, which has the advantage that the melt mixingis then a continuous process and will also serve to transport the meltto the next downstream stage of the process. If appropriate (as willusually be the case, for example, if an extruder is used as melt-mixingdevice) the components of the powder coating composition may bepre-mixed at ambient temperature before being fed to the melt-mixingdevice.

In one form of process according to the invention, the moltencomposition mixture may be formed in a mixer arranged immediatelyupstream of the atomiser. Examples of mixers which may be used in such aprocess include high-efficiency mixers such as static andcavity-transfer mixers. Individual components of the composition (ormixtures of components) may, for example, be fed to the mixer by way ofa corresponding plurality of separate feed means. Each such feed meansmay conveniently be an extruder. Such a process has the advantage ofpreventing or reducing unnecessary contact between co-reactivecomponents of the composition.

In a further form of process, there is a common feed of moltencomposition to a plurality of atomisers. As another possibility, aplurality of atomisers may be supplied by a corresponding plurality ofindividual feed means, and the composition supplied to each atomiser maythen be the same or different. Instead, two different thermosettingcompositions may be fed separately to a single atomiser.

Another possibility comprises simultaneous atomisation of two individualcompositions into a chamber in which the atomised compositions are mixedand cooled.

The atomising step of the process of the invention may for example becarried out by rotary atomisation in which the molten powder coatingcomposition impinges on the surface of a rotating member, for example adisc or cup, and atomisation is effected by accelerating a film ofliquid by centrifugal energy. The rotating member can, for example, be asharp-edged flat disc, a shallow inverted bowl, a vaned disc, or a cupwithout the diverging walls at its open end. A rotating disc may beprovided with a plurality of holes (for example, in the form of slots)or may be in the form of a porous barrier or mesh through which theliquid passes, or may be formed with a serrated edge to encourage theformation of ligaments which break down to form atomised dropletsproducing particles of the desired size. A rotating cup may be mountedon an inclined access, for example at an angle of 15-45 degrees to thehorizontal. A preferred rotary cup is of generally conical form, whichhas been found to give an improved particle size distribution in theproduct, as compared with the products of conventional comminutionprocesses, in that the atomisation process of the present invention canbe conducted so as to restrict very significantly the proportion ofproduct particles that are outside the range of 10 to 120 microns thatis generally required for electrostatic spray application of powdercoating compositions, thereby avoiding the need for subsequent sizeclassification operations.

In a preferred process, the rotary atomisation is carried out with arotating cup or bowl, and the molten mixture is introduced into the cupor bowl through a nozzle which terminates at a position below the levelof its upper rim to promote steady liquid feed to the centre of the cupor bowl. Such an arrangement has been found to give improved results interms of the particle size distribution in the solid end product and interms of increasing the proportion of spherical or near-sphericalparticles.

Rotary atomisation in accordance with the invention may be assisted by agas blast (normally introduced in a direction parallel to the axis ofrotation of the rotary member) or by electrostatic action.

The atomising process of the invention can alternatively be carried outby so-called two-fluid atomisation in which a high velocity gas streamimpinges on a flowing molten powder coating composition to causeformation of an unstable diverging film of the composition, and henceatomisation. The high velocity stream is usually air but, provided thatthe composition is sufficiently resistant, could be steam to provide anauxiliary heating effect. When air or other gas is used the coolingeffect of expansion of the gas on leaving a high velocity nozzle wouldnormally be compensated by some additional heating of the air or gas.The molten powder coating composition can emerge in the form of a jetfrom an orifice into a high velocity gas stream or the moltencomposition can flow as a film over a surface before the gas impinges onit. The direction of gas flow can be substantially linear, whichgenerally results in the formation of a flat spray of atomised droplets,or the gas can have a swirling motion which results in the formation ofa conical spray of atomised droplets. Examples of two- fluid atomisersare shown in FIGS. 1g to 11 of the paper "Effects of Airblast AtomiserDesign upon Spray Quality" by A. K. Jasuja (AGARD paper CP-353). Asindicated above, rotary atomisation can be combined with two-fluidatomisation, for example the molten polymer composition can impinge on arotating disc with high velocity air impinging on the molten film at theedge of the disc, or an apparatus of the type described in DE-A-3326831can be used.

In a further form of process according to the invention the atomisationis ultrasonic atomisation. Ultrasonic atomisation uses vibrations orsound of frequency at least 18 kHz, for example 20 to 80 or 100 kHz,especially 20-50 kHz. The molten powder coating composition can, forexample be caused to flow over the surface of an ultrasonic transducerhorn. Alternatively ultrasonic standing wave atomisation can be used inwhich the molten powder coating composition is caused to flow into thetuned air or gas filled gap between either one ultrasonically vibratingtransducer horn and a reflector or two opposed ultrasonically vibratingtransducer horns. The melt is preferably delivered to the velocityantinode of the standing wave. Ultrasonic atomisers are described in thepaper "New Developments of Ultrasonic Atomisers" presented by L. Bendigat the 4th International Conference on Liquid Atomisation and SpraySystems held in Sendai, Japan, August 1988 (pages 133 to 137 ofConference papers). In particular a standing wave atomiser is describedon pages 136 to 137 with reference to FIGS. 11 and 11a. Standing waveatomisers are also described in DE-C-2656330, DE-C-2842232 andEP-A-308600 and such atomisers may be used in the process of theinvention. A Hartmann whistle atomiser or twin fluid ultrasonic atomiseruses a high velocity air jet impinging on the open end of a smallcavity. Strong oscillating shockwaves are produced in the space betweenthe cavity and the nozzle producing the air jet; the molten powdercoating composition is fed to this space.

With regard to the particle size of the product, the relevant parameterswill in general depend on the method of atomisation used. Thus, forexample, in the case of rotary atomisation the production of particlesof smaller particle size will in general be favoured by increasing thespeed of rotation of the rotary member, by decreasing the flow rate ofthe molten composition into the atomiser, and by increasing the diameterof the rotating member.

In the case of ultrasonic atomisation, relevant parameters affectingparticle size include the flow rate of the molten composition, thedensity and surface tension of the molten composition, and the acousticfrequency employed.

In general, it is considered that simple pressure atomisation (forexample, using a swirl-jet atomiser) will not be suitable for use withmolten thermosetting materials in accordance with the invention, and theatomisation stage of the process of the invention is preferablyconducted other than by such atomisation. This arises because suchconventional atomisers rely essentially on the production of a finespray by the propulsion of a liquid at high velocity through a smallaperture. Such small nozzle apertures are susceptible to erosion damageand to blocking, and those disadvantages would be expected to beespecially serious in the case of a molten thermosetting powder coatingcomposition.

Once the composition has been melt-mixed, it should be maintained in themolten state continuously until atomisation into droplet form iscomplete and there is advantageously no intervening cooling. Also, toprepare the molten composition for atomising, it will in general benecessary for it to undergo further heating to raise its temperatureabove the temperature at which the composition leaves a melt-mixingapparatus. Such further heating is preferably carried out in a rapid andpenetrative manner, for example, by radio-frequency, microwave orvibrational energy input. Where heating is by more conventionalconduction means those skilled in the art will appreciate the advantageof increasing the heat transfer efficiency by increasing where possibleeither or both the contact area of the heat exchanger and the turbulenceof the liquid flow for example by the use of static mixing elements.This heating process is prefer ably applied to the molten compositionimmediately prior to atomisation or additionally or alternatively withinthe atomisation device such as, for example, by heating the cup of arotary atomiser with hot air, induction, infra-red or by more than onesuch expedient. The purpose of the further heating is to reduce theviscosity of the melt in order to facilitate or permit atomisation,without at the same time causing unacceptable pre-reaction of thecomposition.

The viscosity of a thermosetting powder coating composition at normalextrusion temperature may be as high as 5000 poise, and it will ingeneral be necessary to effect a substantial reduction in viscositybefore atomisation. Typically, the viscosity of the molten mixtureimmediately prior to atomisation may be in the range of from 2 to 120poise, advantageously from 5 to 100 poise, preferably from 5 to 50poise, and more especially from 5 to 30 poise.

The surface tension of the molten mixture immediately prior toatomisation may lie within the range of from 20 to 75 dyne.cm, forexample, 30 to 40 dyne.cm. In general, within the indicated ranges, ahigher surface tension will favour the production of spherical andnear-spherical particles, whilst a lower surface tension will tend tofacilitate the atomisation process itself. Accordingly, there will inpractice be a balance between those conflicting factors.

Typically, in order to limit undesired pre-reaction, the mixedcomposition will leave a melt-mixing device such as an extruder at atemperature not exceeding 160° C., for example, in the range of from 90°to 150° C. although the temperature may be as high as 180° C. in thecase of certain compositions, for example, a catalyst free feed. Ingeneral, the molten mixture will need to be heated to a temperaturewithin the range of from 100° to 300° C. immediately prior toatomisation, preferably from 140° to 250° C., more especially from 160°to 240° C. In principle, within the indicated temperature ranges, it isdesirable for the molten composition to attain as high a temperature aspossible immediately prior to atomisation. At the same time, however, itis essential to avoid unacceptable pre-reaction of the thermosettingcomposition, and accordingly there is a balance between the attainablerate of temperature increase and the maximum heat input that can inpractice be tolerated prior to atomisation.

It will be appreciated that the temperature profile used in anyparticular case will depend primarily on the nature of the thermosettingcomposition.

The time within which significant thermosetting of the compositionoccurs is a function of the heating process and also varies with thechemical composition of the thermosetting polymer and the crosslinking(curing) agent, and also according to whether any catalyst is used. Byway of illustration, in the case of a carboxylic acid-functional polymerused with an epoxy functional crosslinking agent, significantcrosslinking occurs at 160° C. after about 20 seconds and at 200° C.after about 5 to 10 seconds. As will be appreciated by those skilled inthe art, if cross-linking were to occur to any substantial extent, thegel time of the product composition would be correspondingly reduced,with consequential adverse effects on flow and levelling properties andoverall detraction from the appearance of a coating formed from thecomposition by a powder coating application process. With increasingcross-linking, the effect on gel time would eventually be such that theproduct composition would not be regarded as suitable for use in powdercoating application processes. As an indication of what may be regardedas significant cross-linking, it may be expected that some deteriorationin the visual appearance of the coating would be apparent if the geltime were to fall by 20%, that some deterioration in the mechanicalproperties of the coating would be evident if the gel time were to fallby 40%, and that a material with a gel time reduced by 50% (as comparedin each case with the gel time of a powder formed from the same initialcomposition by conventional comminution processes not involving meltatomisation) would for practical purposes be unusable for powder coatingapplications.

In the process of the invention, the atomised droplets are caused orallowed to cool to form solid powder particles. Preferably, in order tominimise adherence of the particles to any surface of the apparatus, thetemperature of the atomised particles is reduced to a temperature belowthe softening point of the polymer composition (typically 50° C. orbelow) before the particles encounter any such surface.

By way of example, the rate of cooling may be such that the atomisedparticles have cooled to ambient temperature within about 0.5 secondsafter formation. Although a relatively rapid rate of cooling ispreferable in that it enables a smaller collection vessel to be used, itis also important that the rate of cooling should not be too rapid,otherwise there will be an increasing tendency for the atomised dropletsto be of acicular rather than spherical form. For this reason, it may beadvantageous to arrange for the material under going atomisation to beexposed to a zone of hot air or other gas, with the object of providinga relaxation period in which the composition--although cooling--is stillmaintained in molten form. The temperature of introduction of such gasinto the atomising vessel may be in the range of from 150° to 350° C.,for example, 200° to 250° C.

The velocity of introduction of such hot gas into the atomising vesselshould preferably be so chosen as to approximate generally to the flightvelocity of the atomising droplets, with the object of entraining thedroplets whilst minimising the effects of drag on the droplets whichmight adversely affect the particle shape of the product. Thus, forexample, the gas velocity may typically be in the range of from 5 to 10meters/second at the onset of droplet formation (that is, at theperiphery of the rotating member in the case of rotary atomisation)decelerating to about 1 meter/second at the end of the relaxation zone.

A relaxation zone as described above may be produced by introducing ahot gas so as to impinge on the disintegrating material produced by theatomiser along the direction of droplet flight. Thus, for example, inthe case of a rotary atomiser, hot air is advantageously delivered frombelow as well as from above the plane of the atomiser, so that theresultant flow of hot gas is generally along the said direction.

In atomisation processes involving a stream of high velocity air or gasof which the rapid expansion causes a cooling effect, it may benecessary to provide additional heating to control the cooling rate ofthe droplets produced. In other atomisation processes a current of acooling fluid can contact the atomising droplets before they arecollected. The cooling fluid may be gaseous or liquid (for example,water) but is conveniently air. The particles formed can, for example becollected on a moving belt or can fall into a receptacle. Thisreceptacle can be the packaging in which the powder coating is to besold. Alternatively, the powder coating particles may be produced as afluidised stream in air which is passed to a separator such as a cycloneseparator or into a filtration system such as, for example, a so-called"bag house" to recover the product.

Several forms of process in accordance with the invention will now bedescribed, by way of example, with reference to the accompanyingdrawings:

The apparatus of FIG. 1 consists generally of an extruder 1 feeding anatomiser 2 mounted in an atomising chamber 3.

The ingredients of the powder coating composition, that is thethermosetting polymer, the cross-linking agent and any pigments andother additives are pre-mixed and charged to feed hopper 11. Hopper 11feeds extruder 1 which has a screw 12 driven by motor 13. The extruderhas a heating jacket 14 to control the temperature of the polymercomposition in the extruder to a temperature at which the polymer issoftened but not cured, typically a temperature within the range90°-180° C., depending on the composition. The polymer composition isthoroughly mixed by screw 12 and is extruded through die 15 into a feedpipe 16 leading to the atomiser 2.

The feed pipe 16 passes through heater 17 in which the polymercomposition is rapidly heated, for example by circulating hot air or byradio frequency or microwave heating. The temperature of the polymercomposition on exit from the heater 17 can for example be in the range160°-250° C.

The atomiser 2 is mounted within chamber 3. The atomiser 2 can forexample be a rotary cup atomiser such as that of FIG. 2 or that of FIG.3 or can for example be a standing wave ultrasonic atomiser as describedin DE-C-2842232 or EP-A-308600 or a two-fluid atomiser supplied with hotair. The feed pipe 16 terminates in a nozzle or set of nozzles suitablefor the atomiser used. Hot air is fed through inlet 21 to the centralportion 22 of the chamber 3 so that the feed pipe 16 is surrounded byhot air as it enters chamber 3 to avoid premature cooling of the polymercomposition both before and after atomisation. Cold air is fed throughan inlet 23 to the peripheral portion 24 of chamber 3. A screen 25 ispositioned across the chamber 3 above the level of atomiser 2 toencourage the even distribution of hot air in the central portion 22 ofchamber 3 and the even distribution of cold air around the peripheralportion 24 of chamber 3. A circumferentially extending baffle 26prevents the stream of cold air from inlet 23 from impinging directly onfeed pipe 16 or the atomiser 2

The molten polymer composition is atomised by atomiser 2. When a rotarycup atomiser is used the resulting atomised droplets are thrown outwardswithin chamber 3. After an initial temperature relaxation period whilstthe droplets remain under the influence of the hot air curtain from thecentral portion 22, the droplets rapidly solidify into particles ofpowder coating composition as they encounter the cold air entering fromthe peripheral portion 24 of chamber 3. The particles are carrieddownwards in the stream of air to the outlet 27 at the base of chamber3.

When a standing wave ultrasonic atomiser is used it is generallyarranged so that the ultrasonic field is substantially horizontal. Theatomised droplets are projected in all directions normal to theultrasonic field, including upwards. Air is preferably blown downwardsin the region of the ultrasonic standing wave to carry the atomiseddroplets downwards towards the outlet 27 of chamber 3.

When a two-fluid atomiser is used with air as the atomising fluidadditional air is preferably used to cool the droplets and to transportthe product towards the outlet 27 of the atomisation chamber 3. Whensteam is used as the atomising fluid in a two-fluid atomiser water maybe sprayed around the peripheral portion 24 of chamber 3. Alternatively,or in conjunction with such water spray, water may be allowed to fall asa film over the inner surface of chamber 3 so as to collect andtransport the product particles towards outlet 27.

The outlet 27 of chamber 3 leads through duct 31 to a separator 32, forexample a conical cyclone separator. In the separator 32 the particlesof powder coating composition are separated from the air stream. Theparticles of powder coating composition pass downwards to outlet 33controlled by valve 34 at the apex of the separator 32. The outlet 33may directly feed packages for the powder coating composition in productcollection zone 35 or may feed a hopper for subsequent packaging of theproduct. The air passes upwardly to upper outlet 36 of separator 32.Flow of air from the separator can be aided by fan 37. The outlet of fan37 leads to a filter bag 38 which removes any particles of powdercoating composition carried by the air stream through outlet 36. Airpasses through the filter bag 38 to the surrounding atmosphere.

In cases where steam and water are used as described above, the productfrom the chamber 3 will be in the form of a slurry and the productparticles will be recovered by the use of settling tanks, hydrocyclones,filtration, and/or dried as appropriate.

The atomiser of FIG. 2 comprises a cup 41 mounted on a rotatable spindle42. The cup 41 has a flat base 43, for example of diameter about 18 mm,and a concave side wall 44 of height about 25 mm with an upper lip 45 ofdiameter, for example about 30 mm. The feed pipe 16 leading fromextruder 1 (FIG. 1) terminates in a nozzle 46 of generally conical shapeflattened at its tip 47 and having an outlet orifice 48. The nozzle 46is mounted so that its orifice 48 is below the lip 45 of cup 41 but witha clearance of, for example 15 mm between the tip of the nozzle 46 andthe base 43 of cup 41.

The atomiser of FIG. 3 comprises a cup 51 mounted on rotatable spindle42. The cup 51 is of conical shape, for example with an internal depthof about 20 mm and a diameter at the rim 52 of about 35 mm. The nozzle54 at the end of feed pipe 16 for use with cup 51 is of conical shapeand of similar cone angle to cup 51 so that the face of nozzle 54 issubstantially parallel to the side walls 53 of cup 51. The nozzle 54 ispositioned with its outlet orifice 56 below the rim 52 of cup 51. Thetip of nozzle 54 can, for example be about 7 mm from the apex of cup 51.

The speed of rotation of the cup 41 or 51 in the apparatus of FIG. 2 orFIG. 3 can for example be in the range 5,000 to 30,000 r.p.m..

Referring to FIG. 4 of the accompanying drawings, the apparatuscomprises a rotary cup atomiser 441 mounted within a chamber 442provided with an insulating lid 443 and an inspection window 444.

The chamber 442 has an upper hot air inlet 445, a lower hot air inlet446, and a circumferentially extending cold air inlet 447.

The rotary cup atomiser 441 is in the form of an inverted cone and isrotated by a motor 448 driven by compressed air. The atomiser and motorare mounted on a gantry 449, which also supports an annular plate 450which is formed of a thermally insulating material and has a truncatedconical portion 451 depending centrally therefrom.

In operation, a molten thermosetting powder coating composition(prepared, for example, in an extruder) is passed along a feed pipe 452to a nozzle 453. The feed pipe may be provided with a heater (notshown), for example, a tape heater, and there are additional heatingmeans (not shown) arranged to act immediately upstream of the nozzle453.

The upper hot air inlet 445 opens into a manifold which surrounds thenozzle 453. Hot air introduced in this way serves to heat the atomisercup as well as to assist in providing a temperature relaxation zone asdescribed hereinbefore.

The lower hot air inlet 446 opens into the truncated conical portion 451surrounding the conical rotary cup 441. Hot air introduced in this wayserves to heat the atomiser cup as well as to assist in providing atemperature relaxation zone as described hereinbefore.

In operation, atomisation of the coating composition takes place bycentrifugal energy at and immediately beyond the periphery of therotating cup 441. The atomised droplets so formed are thrown outwardsacross the annular plate 450 and are exposed immediately to the hot airflow from the lower hot air inlet 446. The result is to form atemperature relaxation zone across the annular plate 450, therebylimiting the rate of cooling of the atomised droplets before theyencounter the cold air from the inlet 447. The effect of the relaxationzone is to increase the spherical character of the product particles.

The product particles have cooled to a temperature below the softeningtemperature before they encounter the wall of the chamber 442, and leavethe chamber by way of outlet 454 for further handling as described abovewith reference to FIG. 1.

The following series of experiments illustrate the principle of theprocess of the invention:

A series of experiments was carried out on atomising a pigmentedcarboxylic acid-functional polyester of melting point about 120° C. andTg 60° C. using the apparatus of FIGS. 1 to 3. The polyester compositionwas extruded at an exit temperature of 150° C. The chamber 17surrounding the feed pipe 16 was heated by air at a temperature of 250°C. The temperature of the polyester composition after leaving theheating chamber 17 was about 200° C. The results are shown in thefollowing Table:

    ______________________________________                                        Type of cup used                                                                             FIG. 2  FIG. 2  FIG. 3                                                                              FIG. 3                                   Type of nozzle used                                                                          FIG. 3  FIG. 2  FIG. 3                                                                              FIG. 3                                   Cup speed (rpm)                                                                              10000   10000   10000 10000                                    Polymer composition                                                                          6.5     6.5     6.5   2.6                                      flow rate (g/Min)                                                             % of spherical particles                                                                     70      85      65    >95                                      (aspect ratio                                                                 less than 2:1)                                                                Mean diameter of                                                                             85      75      85    75                                       particles (microns)                                                           Maximum diameter of                                                                          150     130     190   125                                      particles (microns)                                                           Minimum diameter of                                                                          45      25      25    30                                       particles (microns)                                                           ______________________________________                                    

The following Examples illustrate the process of the invention.

The following formulations were used, all percentages being by weight:

    ______________________________________                                        Formulation A [AM D-3 and AM D-5]                                                                       Polyester/Epoxy hybrid                                                        TITANIUM DIOXIDE (Pigment) 20.6%                                              BLANC FIXE (Extender) 18.2%                                                   MICROCARB 40* 2.0%                                                            C-MOXY-FUNCTIONAL POLYESTER RESIN 42.3%                                       (Acid Value 33-35)                                                            DER 671 EPOXY RESIN** 16.3%                                                   WAX 0.3%                                                                      BENZOIN (de-gassing agent) 0.3%                                               *MICROCARB 40 is a calcium carbonate extender                                 **DER 671 is a bisphenol-A/epichlorohydrin                                    epoxy                                                                         resin                                                                         Epoxylation B [AM D-6]                                                        TITANIUM DIOXIDE (Pigment) 35.0%                                              DOW DER642U EPOXY RESIN* 61.2%                                                BYK 36OP FLOW AID 0.4%                                                        DEH 40 CROSSLINKER** 3.1%                                                     BENZOIN (degassing agent) 0.3%                                                *DER 642U is a bisphenol-A epoxy/novolak resin                                **DEH 40 is a dicyandiamide epoxy curing agent                                Polyester/Isocyanate7]                                                        TITANIUM DIOXIDE (Pigment) 33.3%                                              HYDROXY-FUNCTIONAL POLYESTER                                                  (Hydroxyl number: 38-45)                                                      VESTAGEN B 1530 (Curing agent)* 13                                            BYK 630P (Flow aid) 0.4%                                                      BENZOIN (de-gassing agent) 0.2%                                               *VESTAGEN B 1530 is a caprolactam-blocked                                     isocyanate curing agent                                                       Polyester/PrimidM D-8]                                                        CARBOXY-FUNCTIONAL POLYESTER 93.5%                                            (Acid Value: 35-37)                                                           PRIMID XL 552* (Curing agent) 5.2%                                            MODAFLOW (Flow aid) 1.0%                                                      BENZOIN (de-gassing agent) 0.3%                                               *PRIMID XL 552 is a hydroxyalkylamide curing                                  agent                                               ______________________________________                                    

II. Experimental procedure

In each experiment the ingredients of the corresponding formulation werepre-mixed in a blender and metered into a single-screw extruderoperating at between 70° and 130° C. so as to produce a steady outflowof the composition in molten form. The molten material was piped to anapparatus as shown in FIG. 4 by way of feed pipe 452.

The nozzle 453 of the atomising device of the apparatus was maintainedat a temperature of 350° to 400° C., resulting in the molten materialleaving the nozzle being at the feed temperature shown in Table 1 below.The speed of rotation of the atomising cup 441 was in the range of10,000 to 15,500 as shown in Table 1. Hot air was introduced through theupper air inlet 445 and the lower air inlet 446 at the temperaturesshown in Table 1.

The product powder was analysed by standard techniques and the resultsare shown in Table 2 below.

                  TABLE 1                                                         ______________________________________                                                  Air temperature                                                                         Cup      Feed    Feed                                     Expt.  Formu-   Upper   Lower rotation                                                                             rate  Temp                               No.    lation   °C.                                                                            °C.                                                                          × 1000                                                                         Kg/h  °C.                         ______________________________________                                        1.1    A        200     200   12     1.0   200                                1.2    A        250     250   12     1.0   210                                2.1    A        250     180   15     1.0   230                                2.2    A        250     230   15     3.0   210                                3.1    B        205     202   15.5   2.0   160                                3.2    B        190     190   15     3.0   160                                4.1    C        190     125   14.7   3.0   210                                4.2    C        235     235   14.8   3.0   210                                5.1    D        214     210   15     2.0   190                                5.2    D        230     238   15     3.0   210                                ______________________________________                                    

In order to provide a basis for comparison, Table 2 also shows (marked"S") the results of the same analyses conducted on powder coatingcompositions corresponding to formulations A to D but prepared byconventional micronising procedures.

                  TABLE 2                                                         ______________________________________                                                       Mean                                                                          Particle Tg    Gel    Pellet                                                                              Gloss                              Expt. Formu-   Size     Onset Time   Flow  (60°)                       No.   lation   Microns  °C.                                                                          Seconds                                                                              mm    %                                  ______________________________________                                        1s    A        39.1     49.1  156    66    85                                 1.1   A        64.6     51.3  162    44    87                                 1.2   A        66.3     51.1  143    47    84                                 2s    A        38.1     49.1  156    66    85                                 2.1   A        64.1     45.2  148    47    88                                 2.2   A        70.0     44.1  151    57    87                                 3s    B        34.3     49.2   11    55    86                                 3.1   B        70.5     55.4   2      9    48                                 3.2   B        73.0     53.4   13    53    87                                 4s    C        35.8     43.5  133    77    80                                 4.1   C        83.0     50.7  142    73    79                                 4.2   C        73.2     42.5  138    68    75                                 5s    D        37.2     61.7   88    103   82                                 5.1   D        63.8     60.2   83    61    52                                 5.2   D        55.5     61.7   80    102   90                                 ______________________________________                                    

III. Analytical Techniques (a) Particle Size

The particle size data are based on observations made using a GalaiCis-1 particle size analyser using surfactant and ultra-sound fordispersion of the sample in water.

(b) Tg

The Tg data are based on observations made using a Du Pont 910differential scanning calorimeter with a Du Pont 2000 thermal analyser.

(c) Gel Time

The gel time is the time at which the product appears to become anelastic solid when heated at 200° C.

(d) Pellet Flow

For this test a pellet of volume 0.47 ml is formed from the productcomposition and is pressed on to a metal plate inclined at an angle of60° C. The plate bearing the pellet is then heated in an oven at 150° C.for 30 minutes, and the distance through which the composition hasflowed down the plate in that time is measured in mm.

(e) 60° Gloss

The 60° gloss data are based on observations using a LaborReflectometer.

IV. Discussion

In terms of the various relevant parameters, and especially gel time,the results set out in Table 2 show, with the exception of Experiment3.1, that the product compositions obtained by melt atomisationprocesses according to the invention were acceptable for use in powdercoating application processes.

Following the unsatisfactory result obtained in Experiment 3.1, theprocess conditions were varied, in Experiment 3.2, by reducing the airtemperature and increasing the feed rate of the molten material to theatomising device, thereby reducing both the residence time of thematerial in the device and the total heat input to the material duringthe process. As a result of taking those measures in accordance with thegeneral information given hereinbefore concerning the practice of thepresent invention, it will be seen that a satisfactory product was thenobtained using the same composition formulation as in Experiment 3.1.

V. Molecular Weight Studies--Gel Permeation Chromatography

The following Table 3 shows the number average molecular weight [Mn],the weight average molecular weight [Mw], the volume average molecularweight [Mz] and the dispersity factor [ratio of Mw:Mn] as determined bygel permeation chromatography for the products of Experiments 2.1 and2.2 as described above, together with the corresponding data for thestandard comparison material [designated "2s"] of the same formulation.

                  TABLE 3                                                         ______________________________________                                        Experiment                                                                    No.        Mn     Mw        Mz    Dispersity                                  ______________________________________                                        2s         1483   7753      18614 5.3                                         2.1        1283   7773      18130 6.1                                         2.2        1446   7881      20120 5.5                                         ______________________________________                                    

It is considered that, within the limits of experimental error, thefigures given in Table 3 show no significant variation in molecularweights as between the standard comparison product and the correspondingproducts obtained by the melt atomisation process of the invention.

VI. Assessment of Particle Shade

FIGS. 5 and 6 of the accompanying drawings reproduce respectivelyelectron micrographs of a sample of the product of Experiment 1.1 inaccordance with the invention and of the corresponding standardcomparison material (designated "1s").

It is apparent from the electron micrographs that the product obtainedin accordance with the invention (Experiment 1.1, FIG. 5) shows a muchreduced content of fine particles and a narrower size distributiongenerally, as compared with the standard comparison product obtained byconventional comminution processes, and also contains a high proportionof particles of a rounded shape.

We claim:
 1. A process for the manufacture of a thermosetting powder coating composition, which comprises:(a) forming a molten mixture of a film-forming polymer containing reactive functional groups, a curing agent for said polymer and optionally one or more coloring agents, the composition being formed without addition of solvent and having a residual solvent content not exceeding 1.0% by weight derived from manufacture of any of the ingredients; (b) atomizing the resulting melt into droplet form, the temperature of the molten mixture immediately prior to atomization being in the range of from 100° to 300° C.; and (c) causing or allowing the atomized droplets to cool to form solid powder particles,whereby significant thermosetting of the composition during the process is avoided such that the gel time is reduced no more than 40% as compared with the gel time of a powder formed from the same composition prepared by a comminution process, and the product composition is suitable for application as a powder coating.
 2. A process according to claim 1, wherein the melt is atomised by rotary atomisation.
 3. A process according to claim 2, wherein the rotary atomisation is carried out with the use of a rotary cup of generally conical form.
 4. A process according to claim 2, wherein the rotary atomisation is carried out with a rotating cup or bowl, and the molten mixture is introduced into the cup or bowl through a nozzle which terminates at a position below the level of its upper rim.
 5. A process according to claim 1, wherein the melt is atomised by two-fluid atomisation.
 6. A process according to claim 1, wherein the melt is atomised by ultrasonic atomisation.
 7. A process according to claim 1 wherein the composition undergoing atomisation is exposed to a current of a hot gas.
 8. A process according to claim 1 wherein one or more currents of hot gas, impinge on the composition undergoing atomisation along the direction of droplet flight.
 9. A process according to claim 1 wherein after melt-mixing the composition is maintained in the molten state continuously until atomisation into droplet form is complete.
 10. A process according to claim 9, wherein the molten composition does not cool to any substantial extent between melt-mixing and atomisation.
 11. A process according to claim 1 wherein the molten composition is subjected to further heating immediately prior to atomisation.
 12. A process according to claim 11, wherein the said further heating is effected by radio-frequency, microwave or vibrational energy input.
 13. A process according to claim 1 wherein the temperature of the molten composition immediately prior to atomisation is in the range of from 140° to 250° C.
 14. A process according to claim 1 wherein the composition is melt-mixed in an extruder.
 15. A process according to claim 1 wherein the temperature of the molten composition leaving the melt-mixing device does not exceed 160° C.
 16. A process as claimed in claim 1 wherein a catalyst for the curing section is injected directly into the atomiser, so as to be mixed with the molten film-forming polymer before the onset of droplet formation.
 17. A process as claimed in claim 1 wherein at least a proportion of the curing agent is injected directly into the atomiser, so as to be mixed with the molten film-forming polymer before the onset of droplet formation.
 18. A process as claimed in claim 1, wherein the film-forming polymer is selected from the group consisting of carboxy-functional polyester resins, hydroxy-functional polyester resins, epoxy resins, and fictional acrylic resins.
 19. A process according to claim 1 wherein the curing agent is itself a polymeric material.
 20. A thermosetting powder coating composition made by a process as claimed in claim
 1. 21. A process according to claim 7 wherein the hot gas is air.
 22. A process according to claim 8 wherein the hot gas is air.
 23. A process according to claim 13 wherein the temperature of the molten composition immediately prior to atomisation is from 160° to 240° C.
 24. A method of forming a coating on a substrate which comprises the steps of:(1) applying to a substrate by a powder coating process a thermosetting powder coating composition prepared by (a) forming a molten mixture of a film-forming polymer containing reactive fictional groups, a curing agent for said polymer and optionally one or more coloring agents, the composition being formed without addition of solvent and having a residual solvent content not exceeding 1.0% by weight derived from manufacture of any of the ingredients; (b) atomizing the resulting melt into droplet form, the temperature of the molten mixture immediately prior to atomization being in the range of from 100° to 300° C.; and (c) causing or allowing the atomized droplets to cool to form solid powder particles, whereby significant thermosetting of the composition during the process is avoided such that the gel time of the powder is reduced no more than 40% as compared with the gel time of a powder formed from the same composition prepared by a comminution process; and (2) heating the thus applied composition to melt and fuse the particles on the substrate and cure the coating. 